After nearly two years of fighting, Syria’s vaunted war machine is showing serious cracks as emboldened rebels snap up more bases and airfields and force army units to retrench behind defensive lines in major cities, Western officials and military analysts say.

Bolstered by a steady flow of arms from foreign backers, opposition forces have scored a series of tactical victories in the Damascus suburbs in recent days and are advancing steadily toward the city’s airport, adding to what some analysts view as a sense of momentum that has been building since late summer.

Powerful antitank and antiaircraft weapons have helped level what was once a lopsided contest, the officials say, so much so that army commanders have been unable or unwilling to challenge rebel assaults on large military bases on the capital’s outskirts.

“The regime isn’t intervening to defend its positions,” said Jeffrey White, a former Middle East military analyst with the Pentagon’s Defense Intelligence Agency. “And when it does try to counterattack, it often fails.”

Extremist groups among the Syrian opposition are responsible for some of the gains. Rebel commanders and outside analysts say the groups have grown morepowerful in recent months because of funding and weapons from wealthy Arab donors in the Persian Gulf region as well as Syrian businessmen outside the country.

One Islamist militia with suspected ties to al-Qaeda has seized two government military bases in the past two weeks.

Several independent military experts have pointed to a perceptible shift in the rebels’ fortunes beginning in mid-November, around the time reports began to surface of Syrian helicopters and planes being shot down by shoulder-fired missiles. Western and Middle Eastern intelligence officials say up to 40 of the portable antiaircraft missile systems have been smuggled into rebel-held parts of Syria since late summer.

But analysts say the opposition’s successes also reflect the degraded state of the Syrian army, which appears to be running low on supplies and morale. White, the former DIA analyst, said the rebels “are getting better, with better equipment and more of it, but it’s also true that the government’s troops are being worn down.”

Military experts cautioned that the fighting is likely to drag on, barring a surprise development such as the assassination or abdication of Syrian President Bashar al-Assad. Joseph Holliday, a former U.S. Army officer and senior researcher at the Institute for the Study of War who has examined the capabilities of Syrian rebels, said a decisive victory could be several months, if not years, away.

Holliday said rebel squads have shown increasing tactical skills and deployed momentum-changing weapons, including roadside bombs and antiaircraft missiles. The bombs have limited the movement of Syrian troops and the antiaircraft guns have forced Syrian pilots to fly at higher altitudes, he said. The net result is that the Syrian military has surrendered critical territory and appears to lack the resources to regain ground.

“What we’re seeing is a contraction from the regime,” Holliday said. “The rebels have been successful in forcing the regime to give up on outlying outposts.”

But the rebels continue to suffer from poor coordination among the factions that have taken up arms against the regime across the country, he said.

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

“What we haven’t seen is any organization above the provincial level, and that is concerning,” he said.

Obama administration officials have expressed concern about the absence of a united front among the opposition groups. The administration is expected to join Britain, France and other allies next week in recognizing a newly formed coalition as the legitimate representative of the Syrian people. But Washington remains unwilling to provide arms to the rebels.

Competition for weapons and money has led to a deepening of the rift between Islamist and secular groups within the opposition. Secular commanders complain bitterly of the lack of tangible support from their Western backers.

“The lack of support by the international community has led to a situation where support is coming from the gulf states and from Syrian businessmen in those states,” Col. Malik Kurdi, a spokesman for the rebel Free Syrian Army, said in an interview. “These are people who have the ideology of Salafis and the Muslim Brotherhood. They started supporting groups who have the same ideology in Syria, and some adopted this ideology to get financial support.”

At least half a dozen religious extremist groups have sprung up in Syria since the beginning of the year. The group that has captured the most attention and appearsto have had the greatest degree of success is Jabhat al-Nusra, which is thought to have links to al-Qaeda.

Since the group was formed in January, it has asserted responsibility for a series of suicide attacks against military and security targets. Its forces have overrun at least two government military bases in the past two weeks, collecting weapons left behind by Syrian troops, opposition activists said.

“Unfortunately, there are more extremist groups receiving arms,” said Andrew Tabler, a Syria expert at the Washington Institute for Near East Policy. “That’s changing with the capture of new weapons but also the supply of weapons from outside.”

Despite the steadily increasing flow of arms into the country, some rebel fighters say most of their guns and ammunition come from army bases the rebels have overtaken. Supplies often run short.

In the Turkish border town of Reyhanli, where many wounded rebels go to recover, a 21-year-old man who gave only his first name, Ammar, arrived 11 days ago after a sniper’s bullet sliced through his shin during fighting near Aleppo. He said he had just one rocket remaining in his grenade launcher when he tried to take out the sniper. When the sniper fired back, Ammar was hit in the leg.

“We lack many things,” said Ammar, who defected from the army seven months ago. Now he shares a room with two rebels who worked at an electronics store and a potato chip factory before the war.

“We had no machine guns,” he said. “No antiaircraft weapons. We didn’t have enough ammunition. Just Kalashnikovs” and rocket-propelled grenades.

Down the hall, Mustafa Akush lay in a bed, paralyzed from the waist down since a bullet struck his spine during fighting three months ago in Aleppo. He was a lieutenant in the army, and before he defected early this year, he smuggled ammunition out to resupply the rebels. He said he fled the army officially when he was ordered to report to investigators about the missing bullets.

“There’s a big gap between the weapons the army has and what we have,” Akush said as his brother rewound the bandages covering both his legs below the knees.

He offered only one explanation for the rebels’ success despite being outnumbered and outgunned.

“It’s because we are defending what we feel is righteous,” Akush said.

Dehghanpisheh reported from Beirut. Suza Haidamous in Beirut, Carol Morello on the Turkish-Syrian border and Ernesto Londoño in Washington contributed to this report.

Joby Warrick joined the Post’s national staff in 1996. He has covered national security, intelligence and the Middle East, and currently writes about the environment.

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